Roll grinders and roll lathes are known as machine tools for machining a roll.
The roll lathe is a turning machine in which a cutter rest equipped with a diamond tool is disposed on a carriage. A general use of the roll lathe is to machine a circumferential groove in a roll by rotating the roll on a headstock while moving the carriage in a fore and aft direction (X axis). When a groove is machined in an axial direction, the carriage is moved at a high speed in a right and left direction (Z axis), while dividing (indexing) the roll on the headstock (C axis). In this manner, a groove can be formed in the axial direction.
In recent years, due to an advanced machine control technique, there has been performed a super high-precision machining process for machining a rugged pattern or the like on an outer circumferential surface of a roll. For example, even a die for molding an optical lens can be recently machined by a lathe. The Applicant of the present application has proposed a vertical lathe for machining a die for molding a Fresnel lens (JP2004-358624A). This vertical lathe can precisely machine a V-shaped lens groove in a Fresnel-lens molding die.
With the prevailing use of a liquid crystal display, there is an increasing demand for a lens sheet in use for a back light of a liquid crystal panel. A lenticular lens sheet, a cross-lenticular lens sheet, a prism sheet, as well as a Fresnel lens sheet are used as such a lens sheet.
It has been under review in these days that a lenticular lens sheet, a cross-lenticular lens sheet, and a prism sheet are formed by extrusion molding with the use of a transfer roll.
A transfer roll for a lenticular lens sheet is formed by precisely machining circumferential grooves at predetermined intervals in the outer circumferential surface of a roll. Thus, the machining process can be performed by a roll lathe.
On the other hand, in a transfer roll for a cross-lenticular lens sheet or a transfer roll for a prism sheet, it is necessary to machine a triangular-pyramid pattern or a quadrangular-pyramid pattern in the outer circumferential surface of a roll.
A conventional precise machining process by using the aforementioned precision roll lathe is as follows. At first, a rotational center position of a tool rest is determined by means of a specialized jig. Then, an image of a cutting edge is taken by a microscope (optical system) connected to an apparatus, and the image is displayed on a monitor. Thereafter, an operator manually aligns a cutting-edge position with an intersection of two hairlines, i.e., a vertical hairline and a horizontal hairline, displayed on the monitor. Then, based on coordinates from the rotational center position to the cutting-edge position, information about the cutting-edge position is obtained.
Because of an enlarged size of a die, tools should be replaced in the course of the machining process. At this time, the positioning process for each tool should be performed with a significantly strict precision.
In addition, when a prism sheet or the like is formed, a machining process with the use of one kind of tool may be insufficient depending on an optical design. Namely, as shown in FIG. 13, a prism sheet requires a machining process with the use of plural kinds of tools. In this case, the importance of tool positioning process is further increased. As shown in FIG. 13, when three cutting tolls are used, machining intervals by the respective cutting tools must be extremely precise. Thus, the positioning process for each tool is very important.
In addition, some tool-shape measuring apparatuses, which are capable of being used for specifying a position of a cutting edge, are described in JP2006-284531A and JP8-257876A.